Used to describe the gay huddle that occurs when one (or more) man (usually in a bar or other crowed event) exposes his penis and other men gather around to check it out. A "gun show" is not the exposed penis on its own, but includes the gathering that occurs.

John: "I went to the bar for another beer but got side tracked at the gun show over in the corner. Turns out Sam really does have a huge dick."

Bob: "Johnny loves showing off his big dick."
Peter: "Give him one more beer and I guarantee he'll start a gun show with those guys he's talking too."